Projecting systems used to project still images, motion pictures or visual effects on a projection surface are widely used nowadays in for example cinemas, home cinemas, presentation rooms, exhibition areas, light shows, etc. . . . . In most implementations (see FIG. 1), a projection light source (1) generates a light beam that is guided onto a projection imaging system (2) that will generate the individual pixels containing the colour and brightness of the image to be projected. Then, the light is sent through a projection lens system (3) that will shape, position and focus the projection cone (14) before being projected on the projecting surface (20). This surface can be a flat surface when used for motion pictures or presentation material, but can also be irregular as for example used for the projection of images and motion pictures on buildings. A surface can also be completely absent in the case of light shows
Laser light sources are becoming an attractive projection light source as alternative to traditional lamps and LEDs. They have many advantages over these traditional sources such as a longer lifetime, extended colour gamut and increased brightness making them particular interesting for high brightness projection systems. Also the costs are evolving positively and in the coming years compact low cost laser diodes are expected to become available. Their main drawback however is the stringent safety regulations restricting their use. Laser devices are classified in safety classes according to their wavelength and intensity. Only class 1 and 2 lasers can be used freely. However, for a projection system, the output power of the laser source will exceed the emission levels of these two classes. Therefore a safety system must be included in the projecting system that will detect the presence of a person or an animal in the space where the projection cone can expose the person or animal with excessive laser light. This space that the safety system must at least cover is referred to as the protected space. This can be done be shutting down or reducing the laser light partially or completely. Moreover, a reflective object (21) might be exposed to the projection light (17), reflecting the laser light to a person or animal (22) that is not directly positioned within the projection cone (14). Therefore, also the presence of a reflective object (21) has to be detected even if it is relatively small.
U.S. Pat. No. 6,002,505 discloses a safety detection system for a laser projection system where a detection camera (9) including a sensor or sensor array (7) is used to detect the presence of an object in the laser projection cone (14) by sensing at least the volume of the projection cone, i.e. the protected space. The sensing can be done by different means like thermal measurement, acoustic measurement or electromagnetic measurement. It also includes a detection source (6) used for the irradiation of acoustic or electromagnetic waves. These waves are then reflected by the projection surface or objects and sensed by the detection camera (9), i.e. the sensors. Both the detection camera(s) (9) and detection source(s) (6) are placed nearby the projection system.
In U.S. Pat. No. 6,361,173 discloses a more specific detection system where an infra-red light source is used as detection source (4) and an infra-red camera is used as detection camera (9). Both are placed next to the projection source (1) and used for the detection of a person in protected space. In this application, the detection system is then used to turn off part of the projection light source preventing a person from being blinded.
Both prior art solutions described above have the intrinsic property of being susceptible to blind spots. Close by the lens or light source of the projecting system there will always be spots were an object will not be illuminated by the detection source and hence the detection system will fail. In some embodiments, this drawback has been overcome by putting up multiple detection sources and cameras around the projection source. However, although this limits the blind spot near the projection source drastically, it can never eliminate it. Moreover, one can prove that in the case of a reflective object there will always be blind spots in the reflected area disregarding of the amount of detection sources and detection cameras that are used and disregarding whether the reflective object itself is in a blind spot or not. A second drawback of these prior art solutions is that the detection system has to be adjusted every time the projection source is changed, i.e. when another lens is mounted, when the zoom position of the lens is changed or when the lens is shifted.